Acceptor properties of solvents: the use of isokinetic relationships to elucidate the relationship between the acceptor number and the solvatochromism of N-phenolate betaine dyes

Abstract

The thermochromism of the pyridinium N-phenoxide betaine dyes, betaine 1 and betaine 2, has been studied in different solvents and over a range of temperatures and shown to yield isoparametric (isokinetic) relationships (IKR). (The use of the more lipophilic betaine 2 enabled the range of solvents to be extended to cover very apolar solvents in which betaine 1 is insoluble.) These IKRs led to the identification of five different groups of solvents, each group showing different solute–solvent and solvent–solvent mechanisms and being characterised by their functional groups. The corresponding Tiso values are, as demanded by theory, related to far-IR absorptions that are characteristic of reaction heat baths, which in these systems are found to be the betaines themselves. These groups are reflected in the relationship between Reichardt's ET values and Gutmann's acceptor numbers (AN) which is reinvestigated and each group shown to lie on one of a series of parallel lines in the ETvs. AN plot. It is demonstrated that acceptor parameters depend strongly on the donor properties of the probe used to obtain them.